A diesel particle filter (DPF) is normally coated with a catalyst, which has the function to oxidize the accumulated soot but also to oxidize carbon monoxide (CO) and hydrocarbon (HC). If the loading level of the catalyst in the filter is high enough, the functionality of the upstream positioned diesel oxidation catalyst (DOC) can be integrated into the DPF.
To save space and costs and to improve the efficiency of the whole system, the integration of the selective catalytic reduction (SCR) catalyst into the DPF or even a part of the SCR functionality into the DPF is of great interest. SCR is the reduction of NO and NO2 with NH3 to water and nitrogen according to the following three reactions:4NH3+4NO+O2→4N2+6H2O (“Standard SCR”)2NH3+NO+NO2→2N2+3H2O (“Fast SCR”)8NH3+6NO2→7N2+12H2O (“NO2SCR”)
In the SAE paper SAE 2011-01-1312 a filter with integrated SCR catalyst is described. It is used in combination with additional SCR modules and compared to the corresponding system, where the DPF and the SCR catalyst are separated. The document shows that the system with the combined SCR-DPF has a better performance. In another paper, the SAE 2011-01-1140, a development of a SCR-DPF based on a copper zeolite coated on a cordierite DPF is described. The most important information from this paper is that the soot load level for the SCR-DPF for crack occurrence is 6.8 g/l for the cordierite at a porosity level of 59%. The substrate will survive this soot load but the corresponding temperature inside the filter is >1000° C. and this has an impact on the zeolite.
An example for a wall flow filter which comprises a SCR catalytic active coating inside of its cell walls is given in the WO2011128026A1. In this application a Fe-zeolite coating of about 100 g/l is described and the characteristic feature of the product is that a second over coating is applied on the inlet side of the channels, which reduces deep bed filtration of the soot.
The US2011268635A1 describes another version of a wall flow filter based on a matrix of nonwoven inorganic fibers. This matrix contains a metal exchanged chabazite molecular sieve as the SCR catalyst.
A wall flow filter, where the SCR catalyst is located as a layer on the surface of the outlet channels and a catalyst for oxidizing particulate matter (PM) is located as a layer on the inlet channels, is described in the US2010287915A1. In this solution the two catalytic active coating layers are separated by the cell wall of the DPF.
A catalytic article, which comprises a monolithic wall flow filter, which contain a SCR catalyst composition that permeates the walls at a loading of 1.3 g/in3 is described in the EP 1 663 458 B1.
In the WO2012135871A1 a multifunctional or multicoated wall flow filter is described, which contains all type of coatings: hydrolysis catalyst, SCR catalyst, ammonia oxidation catalyst and oxidation catalyst. The hydrolysis catalyst is located on the inlet side and the oxidation catalyst on the outlet side, both separated by the cell wall of the filter which contains a SCR catalyst. According to the teaching of this document, the separation of the different catalyst types into the different zones is necessary.
A special SCR catalyst composition is described in the WO2013060487A1, where a mixture of zeolite based catalysts with powders based on Ce—Mn/Al2O3 or CeO2—ZrO2 are used to form SCR catalysts. This solution describes how the amount of expensive zeolites in a catalyst composition can be reduced by replacing the corresponding volume by these special metal oxides. A similar solution is also described in the patent application US2009304566A1 and also in the WO2008085280A2. All three documents describe a special type of SCR catalyst composition, which can be used on different types of catalyst supports.
A matrix built up by a zeolite based catalyst powder, -alumina powder, alumina fibers and an alumina sol as the binder for these components is described in the document EP2123355A1. The big disadvantage is, that it is not possible, to make a diesel particle filter based on this composition, because the high amount of alumina produces a high coefficient of thermal expansion and a low thermal conductivity which makes a save regeneration of soot in the filter impossible.
In general, the filter substrates for exhaust gas after treatment systems as described above due to the state of the art have to be highly porous to receive high wash coat loadings. The high porosity levels lead inevitably to very fragile structures, which cause problems in the canning process. It is therefore necessary to find a possibility to reinforce the porous structure before the coating with the functional catalyst, such as SCR catalyst.
To achieve sufficient catalytic activity in respect to the reduction of all 4 components (particle number (PN), carbon monoxide (CO), hydrocarbon (HC) and nitrogenoxides (NOx)) it is necessary to have enough catalyst or catalytic active contact surface in the filter, and this for different type of catalysts. It is therefore necessary to have all type of catalysts distributed homogeneously over the whole filter volume. At the same time, the overall back pressure of the filter should not exceed the value of corresponding conventional filters at lower porosity levels and low catalyst loading levels. All this leads to a restriction for the amount of each type of catalyst, which is coated on the filter.
All the solutions described in the state of the art are focused on after treatment systems which are operating with passive regeneration. But passive operating systems can also run into situations with a slight soot overload and then at higher exhaust gas temperatures this can end up in temperature peaks inside the filter which are not severe for the substrate but for the catalyst. All known SCR catalysts have a temperature restriction of maximum 750° C. In addition to this in the off-road sector there exist applications with the need of active regeneration. It can therefore be beneficial to use a substrate which can buffer this type of temperature peaks.